The present invention relates generally to electronic ballasts for gas discharge lamps. More specifically, this invention pertains to electronic ballasts having circuitry to provide for zero-current switching of the ballast switching transistors.
The operation of a gas discharge lamp requires, among others, a mechanism to control the current delivered to the lamp. Initially, this role was satisfied by magnetic ballasts. However, with the proliferation of integrated circuits, the focus has turned to electronic ballasts to manage the operation of the lamp. The migration from magnetic ballasts to electronic ballasts is due, in significant part, to the increased operational efficiency afforded by electronic ballasts, relative to magnetic ballasts.
The increased efficiency of electronic ballasts is attributable to several factors, one key factor being the use of switching transistors to alter the frequency of the power signals received from the power source, e.g. a standard 120 Volt, 60 Hertz wall outlet, before the power signals are delivered to the lamp. However, using transistors to facilitate power delivery to the lamp can also present efficiency challenges. Namely, for optimal operating efficiency, transistors should be turned on and off, i.e. switched, under little, or more preferably, no load. Thus, if a power signal, comprised of both a current signal and a voltage signal, is present while the transistor is switching, a power loss will occur. This leads to energy inefficiencies and is therefore undesirable. Consequently, switching the transistors when the voltage and current signals are both zero ensures maximum switching efficiency.
Because of the transient and unpredictable nature of the power signal, minimizing switching losses is not an easy task. Numerous strategies have been employed to reduce these losses. Integral to any strategy attempting to address this problem is detecting current and/or voltage signals as at least one must be known for any scheme desiring to reduce switching losses. For example, some strategies are based on monitoring and acting off of the voltage signal alone. Unfortunately, such a strategy has many deficiencies. One such deficiency is that just because the voltage signal may be zero, or at a minimum, does not necessarily mandate that the current signal is also at a minimum. If both current and voltage signals are not at their respective minimum values then the switching efficiency is not maximized. Thus, strategies that rely solely on the state of the voltage level present at the transistor to reduce switching losses are not ideal.
Another strategy is targeted at sensing zero current conditions in the ballast, and more precisely at the switching transistors. Common schemes employing this technique often require floating circuits (circuits without a connection to a common ground or reference voltage). Floating circuits are complex and, accordingly, costly and more prone to failure than non-floating circuits.
Thus, what is needed is a circuit that can detect both zero voltage and current switching conditions in an electronic ballast so that switching losses can be reduced and/or eliminated. Further, it is also desirable that such a circuit would be reliable and inexpensive.